This invention relates to the testing of electrochemical cells during the manufacturing process and, in particular, to the testing of rechargeable type electrochemical cells.
It is accepted practice in the electrochemical cell manufacturing industry to subject each manufactured cell to a test procedure for the purpose of checking cell performance. In the case of electrochemical cells of the rechargeable type, performance is ascertained by measuring the electrical characteristics of the cell after it has been fully charged. The exact nature of these tests varies from one manufacturer to another but, in general, each involves impressing a full electrical charge (or even overcharge) on the cell and, only then, measuring the cell voltage. Thereafter the cell may be discharged to a predetermined level in order to provide a measure of the length of time for discharge or the magnitude of the discharge current.
In one currently followed procedure, rechargeable nickel-cadmium cells are taken from the assembly line and placed in apparatus which charges the cells at a rate that is a fraction of the one-hour power current rating ("C") for the cell. This charging is carried out for a time sufficient to effect at least a certain degree of overcharge to the cell. For example, a typical sealed nickel-cadmium cell is charged at the 0.1C rate for 24 hours (C being the current rating of the cell at one hour). This results in the cell's being supplied with a charge equal to 240% of its rated capacity, and the average normal cell may be in overcharge for perhaps 5-7 hours in order to ensure that all cells subjected to the ensuing tests are completely charged.
Prior to terminating the 0.1C charging current applied into the cell, the cell voltage is measured. This voltage measurement generally provides an indication of an insufficient electrolyte plate mismatch, presence of carbonates or a shorted condition of the cell. A cell voltage while the cell is being charged which is abnormally high indicates a low electrolyte plate mismatch or presence of carbonated condition, whereas a charge voltage which is abnormally low tends to indicate a short.
After this voltage measurement, the cell is subsequently discharged at the rate of 2C (i.e., at a discharge current which is double the one hour current rating C of the cell), and the time needed for the cell to reach a subnorminal voltage is recorded. The discharge time provides an indication of several of the characteristics of the cell, including its capacity, normal or abnormally low electrolyte, leaks, and shorted conditions. If any of the latter conditions exist, the discharge time will be less than the acceptable rating for the cell. This discharge test is founded on the volt-discharge characteristic for the cell. In the case of nickel-cadmium cells, the nominal cell voltage is 1.25 volts. The voltage of a cell which has been fully charged may typically range up to 1.27-1.35 volts. This voltage is relatively constant, dropping to about 1.2 volts when the cell has been discharged by the rated amount and, if discharge continues thereafter, the cell voltage drops abruptly after the voltage reaches about 1.0 volts.
Tests now performed on electrochemical cells, by charging and subsequently discharging the cell, generally provide complete information of the cell behavior and are more than adequate to detect most manufacturing faults or other cell defects; but the procedure has many drawbacks. Space and machinery must be provided in the plant for charging, discharging, and conducting measurements on the cells. Since the cells must be charged for up to 24 hours before any kind of measurement is performed, a great deal of storage space must be provided to accept the full 24-hour plant output of cells so that the cells can be loaded into the charger. It is difficult to conduct such charging on cells on a moving production line due to the large number of cells which are produced over the period of 24 hours (this would require an extremely long track to store and to charge moving cells) and, accordingly, charging is usually done on a batch basis whereby a great number of cells are inserted into the charger at the same time for charging. The second drawback has to do with the discharging of the cell. Adequate electronic circuitry must be provided in order to sense when the cell voltage of each individual cell has attained the predetermined voltage level (e.g. one volt) and then open the discharge path in order that the cell not be put into an overdischarged condition. Discharge of the cell, being at the 2C rate, consumes another thirty minutes of testing time, following which the last series of electrical measurements are made.
From the foregoing, it can be appreciated that a great deal of storage space, testing machinery and time are consumed in the mere testing of the cells owing to the fact that each manufactured cell must be charged and discharged. The testing procedure becomes, in essence, a bottleneck.